David Cameron has threatened Russia with diplomatic, political and economic reprisals in response to Moscow's invasion of Ukraine, with the prime minister telling reporters that Britain had to send a "clear message" to Russia.

Having chaired a meeting of the National Security Council, Cameron said on Monday: "What we want to see is a de-escalation rather than a continuation down the path that the Russian government has taken, violating the sovereignty and territorial integrity of another country. So we shall have to bring to bear diplomatic, political, economic and other pressures in order to make this point. That is the very clear message the whole world needs to send to the Russian government."

Yet despite the prime minister’s strong words, a document being carried into Downing Street appeared to show the extent of Britain's proposed actions against Russia, which would not include trade sanctions. The paper, which was snapped by a photographer, highlighted visa restrictions and travel bans. However, it said: "UK should not support, for now, trade sanctions...or close London's financial centre to Russians".

The document also appeared to suggest that British officials want to "discourage any discussion (eg at Nato) of contingency military preparations", adding that that EU was working on a plan to provide Ukraine with gas should Moscow cut off the supply.

The desire not to impose trade sanctions will be seen as a move to protect Britain's economy, which has close ties with its Russian counterpart. Downing Street declined to comment on the leaked document, and reiterated Cameron's strong stance on the developing situation, adding that any decision will be taken "with EU and G7 partners".

Earlier in Washington, President Barack Obama proffered a similarly stark warning to the Kremlin, calling Russia "on the wrong side of history" in its violation of the sovereignty of Ukraine. The White House is currently putting together its own package of economic and diplomatic measures to isolate Moscow, with the President threatening to make continued military action in Crimea a "costly proposition for Russia".

A member of the self defense unit stands guard as people walk across Kiev's Independence Square

"We all want to see stability and countries obeying the rule of law," said Cameron, adding: "What matters now is that a very clear message is sent from across the world to the people of the Ukraine that we want to allow you to choose your own future; and an equally clear message to the Russian government that continuing down this path of violating the sovereignty of another country will have costs and consequences.

"The whole world needs to come together to deliver that message, it's important that the UN is at the forefront of that message which is why I spoke to secretary general Ban Ki-moon. And we also need to do everything we can to get the Russian and Ukrainian governments to talk to each other about their future."

Cameron's sentiments were echoed by William Hague, who was in Kiev to hold talks with Ukraine's interim Prime Minister Arseniy Yatsenyuk. "If Russia cannot be persuaded to respect the sovereignty and territorial integrity of Ukraine, there will have to be other consequences and other costs," said the Foreign Secretary. "I am not going to set out today what all of those are. We will act in a united way with other nations in the world."

"Russia should be in no doubt about this. This is something that we take very seriously, that we have to take very seriously, because if this becomes the normal way of behaving in the world, of intruding upon and violating the sovereignty of neighbours, then clearly that would be an even bigger crisis in international affairs."

British-based Ukrainians gather outside the Russian Embassy in London

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The Foreign Secretary, who described the situation following the deployment of thousands of Russian troops into the Crimean peninsula as "the biggest crisis in Europe in the 21st century", expressed concern at the potential for further escalation. "Clearly we are very concerned about any possibility of a further move by Russia in other parts of Ukraine," he said.

"There are Ukrainian forces as well as Russian forces in Ukraine. There is a constant risk of miscalculation, of a flashpoint arising there (the Crimea) or in other parts of Ukraine." The Foreign Secretary will update MPs on the situation in a Commons statement on Tuesday.

Speaking at a joint press conference with Hague, Yatsenyuk denounced the Russian incursion as "illegal" and called on Moscow to withdraw its forces. He insisted the new administration in Kiev represented no threat to Russian nationals in the country.

"It is crystal clear that Russia invaded Ukrainian territory with no reason at all. We will protect the rights of minorities," he said. "Every Russian citizen is under the protection of the state of Ukraine. There wasn't anybody who was killed or shot or injured in Crimea except Ukrainian soldiers."

However Russian Foreign Minister Sergey Lavrov insisted Russian troops needed to defend ethnic Russians - who account for almost 60% of the population of Crimea - until "the normalisation of the political situation" in Ukraine. "We are talking here about protection of our citizens and compatriots, about protection of the most fundamental of the human rights - the right to live and nothing more," he said.

Bulgarian protesters shout anti-Russian slogans in front of the Russian embassy in Sofia

Lavrov, who was speaking at the opening a United Nations Human Rights Council session in Geneva, dismissed criticisms of the Russian action claiming Moscow had received information relating to "preparations for new provocations" including against the Russian Black Sea fleet which is based in Crimea.

"Those who are trying to interpret the situation as a sort of aggression and threatening us with sanctions and boycotts, these are the same partners who have been consistently and vigorously encouraging the political powers close to them to declare ultimatums and renounce dialogue,"Mr Lavrov said.

"We call upon them to show a responsibility and to set aside geopolitical calculations and put the interests of the Ukrainian people above all."

It was announced in Brussels that EU leaders will hold an emergency summit on Thursday to discuss the situation in Ukraine.